This invention relates to means for generating a pitch signal which is unaffected by wind shears and turns errors and more particularly to generating a pitch stabilization signal for a head up display which has a long term component derived from an angle of attack signal and a short term signal derived from a gyroscope signal.
Prior art pitch computation circuits are disclosed in Muller U.S. Pat. No. 3,851,303 and Muller U.S. Pat. No. 4,095,271, both assigned to the assignee of this application.
In Muller U.S. Pat. No. 3,851,303, a longitudinal accelerometer signal is modified by a differentiated air speed signal and is combined with a gyroscope signal to provide an indication of the pitch attitude of the aircraft. However, the differentiated air speed signal is subject to wind shear error, thereby decreasing the accuracy of the generated pitch signal.
Muller U.S. Pat. No. 4,095,271, discloses a pitch generator circuit which generates a pitch signal derived from a head up display mounted accelerometer which is calibrated to the display reference axis so that it indicates the pitch angle of the reference axis during steady state, i.e. unaccelerated flight conditions. The accelerometer output is compensated for horizontal acceleration by subtracting the air speed rate to generate a display reference computed pitch signal. This accelerometer derived pitch signal is used as a long term pitch reference and is combined with the gyroscope output for short term pitch excursions. This is accomplished by correcting the gyro pitch signal at a slow and limited rate to the long term reference.
A system disclosed in Greene U.S. Pat. No. 4,012,713, utilizes the output of a longitudinal accelerometer and a differentiated air speed signal to provide a wind shear signal. The signal is fed to appropriate indicator means to alert the pilot or other aircraft of the existence of a dangerous wind shear condition.
These types of systems work satisfactorily under normal conditions or where relatively short term wind shears are present. However, when wind shear conditions of long duration are encountered, the computed long term pitch reference may be in error due to the mismatch between inertial and air mass acceleration, since air mass acceleration is used to compensate for inertial acceleration. In certain wind shear conditions, the computed pitch output may be slowly forced to an erroneous long term pitch reference.
This long term wind shear error can be minimized by reducing the gyro correction slew rate. In so doing, however, gyro erection errors due to slow erection of the gyro platform to the dynamic vertical, as well as errors accumulated during turns cannot be fully compensated. This is especially true during shear conditions where both gyro errors as well as the long term pitch reference errors tend to increase in the same direction, and hence become additive.